Chemical fractionation of organic matter and organic phosphorus extractions from freshwater lake sediment

M R Kurek; M Harir; J T Shukle; A W Schroth; P Schmitt-Kopplin; G K Druschel

doi:10.1016/j.aca.2020.07.013

Chemical fractionation of organic matter and organic phosphorus extractions from freshwater lake sediment

Anal Chim Acta. 2020 Sep 15:1130:29-38. doi: 10.1016/j.aca.2020.07.013. Epub 2020 Jul 26.

Authors

M R Kurek¹, M Harir², J T Shukle³, A W Schroth⁴, P Schmitt-Kopplin², G K Druschel³

Affiliations

¹ Department of Earth, Ocean, Atmospheric Science, Florida State University, Tallahassee, FL, USA; Department of Earth Sciences, Indiana University-Purdue University Indianapolis, Indianapolis, IN, USA. Electronic address: mrk19f@my.fsu.edu.
² Helmholtz Zentrum München, German Research Center for Environmental Health, Research Unit Analytical BioGeoChemistry, Neuherberg, Germany.
³ Department of Earth Sciences, Indiana University-Purdue University Indianapolis, Indianapolis, IN, USA.
⁴ Department of Geology, University of Vermont, Burlington, VT, USA.

PMID: 32892936
DOI: 10.1016/j.aca.2020.07.013

Abstract

Lake sediment organic matter (OM) is composed of a variety of organic compounds differing in their biolability and origin. Sources of sediment OM can include terrestrial input from the watershed and algal/microbial metabolic byproducts residing in the water column or sediment. Dissolved organic phosphorus (DOP) is a critical component of OM in freshwater eutrophic lakes, often acting as a source for bioavailable phosphorus that fuels harmful algal and/or cyanobacterial blooms. Parallel extractions of lake sediment collected from Missisquoi Bay, a eutrophic bay in Lake Champlain, were conducted with the goal of identifying OM and organic P sediment constituents using ultrahigh-resolution mass spectrometry from various extractants. Extractants converged into two groups based on the characteristics of their extracted OM; "stronger extractants" were composed of highly acidic and alkali media, while "milder extractants" represented weaker acids and bases. Sediment treated with the strong extractants afforded highly oxygenated and unsaturated OM thought to be stable with mostly lower heteroatomic content. In contrast, milder extractants yielded highly aliphatic and saturated compounds with lower masses and greater heteroatom functionally, sharing characteristics with labile molecules. Extracted organic P molecules mirrored the bulk OM in terms of lability, mass, and oxygenation within their corresponding extractants. Milder extractants resulted in greater organic P formulae assignments than the stronger extractants, with NaHCO₃ resulting in the most aliphatic organic P formulae. We recommend the use of acetic acid to probe lake sediment for overall molecular characterization, spanning the greatest ranges of O/C and H/C ratios and representing both labile and mineral-associated OM. Other extractants should be implemented for a more targeted analysis. For instance, the use of NaHCO₃ for organic P characterization, while using NaOH when interested in sediment geochemistry; both of which are critical for understanding the factors contributing to internal P loading.

Keywords: FT-ICR MS; Internal loading; Lake sediment; Organic matter; Organic phosphorus; Solid phase extraction.